1. Field of the Invention
The present invention relates to a motor-driven film transporting device for cameras and, more particularly, to a camera having a motor-driven film rewinding device equipped with a timer circuit adapted to automatically bring a film rewind operation to a stop by detecting the film movement state.
2. Description of the Related Art
First, reference will be made to FIGS. 6A, 6B, and 6C to describe phenomena occurring at the time of rewinding of a roll of film loaded into a camera. In a film cassette 100, as shown in FIG. 6A, a roll of film is wound around a spool 100A within the cassette with its rear end attached to the spool and its leader drawn out from an exit slot 100B. In this case, the film 1 will spread up to the inner wall of the cassette 100 because of its elasticity. In this state, the film 1 is drawn out frame by frame with each shot. Finally, the state of FIG. 6C results. When a film rewind operation is performed in this state, the film will be rewound into the cassette 100 immediately. However, in the case of premature rewind in which a roll of film is rewound in such a state shown in FIG. 6A, that is, with some film frames left unexposed within the cassette 100, the film state merely transfers, at the initial stage of the rewind operation, from the state in which the film is spread within the cassette as shown in FIG. 6A to the state in which the film is tightly wound around the spool as shown in FIG. 6B. That is, at the initial stage of the rewind operation, the film portion that is drawn out from the cassette exit slot 100B scarcely moves. It is not until the film enters the state of FIG. 6B that the film portion that is drawn out is taken up into the cassette.
In the following description, such a state as shown in FIG. 6A is referred to as a loosely wound state, while such a state as shown in FIG. 6B is referred to as a tightly wound state.
Heretofore, among cameras equipped with motor-driven film rewinding devices there are cameras that activate a timer in response to the stoppage of movement of a roll of film and stop rewinding the film after a lapse of a predetermined period of time. With such a rewind stopping device using a timer, when the rewind of film is started in the loosely wound state shown in FIG. 6A. the timer runout may occur before the film becomes tightly wound as shown in FIG. 6B, so that the rewinding device comes to a stop before the film is rewound completely. To solve this problem, a longer period of time could be set at the timer. However, this approach would cause the motor to run idly for a long period of time after the termination of a film rewind operation.
In contrast, in Japanese Unexamined Patent Publication No. 1- 279228 there is disclosed a technique of setting a sufficiently long period of time at the timer only in the case of premature rewind. This technique is the result of attention to the fact that the loosely wound state occurs only when the film is rewound prematurely.
U.S. Pat. No. 4,679,926 discloses a technique of setting a long period of time at the timer when film is in the loosely wound state, and changing the period of time set at the timer to a shorter period of time when the film is not in the loosely wound state or when the film has been released from the loosely wound state, these film states being detected in accordance with the movement of film.
However, the technique disclosed in the Japanese Unexamined Patent Publication can prevent the long idle running of the motor at the time of completion of a film rewind operation in the case of autorewind but cannot prevent the long idle running in the case of the premature rewind.
Taking the usage of cameras into considerations, the premature rewind is often performed when film has to be replaced with another as soon as possible. Thus, the long idle running of the motor is not desirable.
On the other hand, with the approach disclosed in U.S. Pat. No. 4,679,926, which changes a period of time set at the timer to another in response to the detection of movement of film, the following problems will arise. That is, film is unstable in the loosely wound state, and thus a phenomenon may occur in which a small number of film-movement signal is output at the same time the film begins to move immediately after the start of rewinding of the film, and the signal ceases to be output very soon. This phenomenon is liable to occur at low temperatures at which the stiffness of film is high, in particular.
In addition, when the film stays in the neighborhood of the threshold of the film movement detecting device, in other words, when, in the case where the film movement detecting device is constructed from a photo reflector, the film is stopping in such a position that detecting light falls on the edge portion of a sprocket hole of film or its neighborhood, a film movement signal may be output by means of noise resulting from idle run of the motor.
In conclusion, in the case of the latter technique as well, if a small number of pulses are output at the beginning of a film rewind operation, and, after that, the winding up of film is continued under the situation in which the output of the film movement detecting device is unstable during the transition from the loosely wound state to the tightly wound state, a period of time set at the timer will be changed to a shorter period of time by those pulses. Therefore, the rewinding device will automatically be brought to a stop while the film is being wound up.